1. Field of the Invention
This invention relates to a method for detecting disturbances which disrupt the lateral continuity of a waveguide. More particularly, this invention relates to a method for the detection of disturbances in a waveguide based on relationships between the properties of layers and bounding surfaces and the propagation velocities and particle displacements of various modes of guided waves that can be expected in underground formations, in particular gas reservoirs.
2. Description of Prior Art
Gas reservoir production is controlled by the architecture of flow units, fractures, sealing elements and bounding surfaces that result from deposition and diagenesis. Differences between the elastic properties of these units and surfaces allow many of them to support the propagation of guided waves. Conventional techniques for delineating the continuity of flow units and sealing and bounding surfaces include crosswell, vertical seismic profiling and surface seismic techniques. A crosswell technique for detecting and analyzing the continuity of subsurface formations in the earth between existing wells is taught by U.S. Pat. No. 5,144,590. In accordance with the teachings of the '590 patent, seismic energy is transmitted from a seismic source at various selected fixed depths in one well and detected as data by a plurality of sensing geophones deployed at selected fixed depths in one or more adjacent wells. A frequency domain decomposition process is performed on the data in order to determine if any of the formations located between the wells function as waveguides for seismic energy within the frequencies of interest. Those formations exhibiting waveguide properties are indicated as continuous between the wells. However, this method utilizes only the time-frequency characteristics of the recorded wavefield which can be misleading for a wide class of waveguide disturbances. In addition, this method will not work if the high frequencies utilized to generate the wavefield couple across the disturbance, a result which we have found, by numerical analysis, often to be the case.
An alternative known method for determining the presence of waveguide disturbances in the earth is seismic crosswell tomography in which seismic energy emitted from sources in one well or borehole is sensed and recorded as seismic data by arrays of detectors in one or more other wells. The recorded seismic data is then processed to form tomographic images of interwell geologic features based on the crosswell seismic data. When used for geophysical purposes, tomographic imaging between boreholes can produce very good images provided certain conditions are present. Unfortunately, in most petroleum and gas reservoirs, these conditions are often very difficult to achieve. In particular, because the imaging solutions are intended to produce 2-dimensional results, the well boreholes must be aligned; that is, they must be vertical or both deviate in the same plane. Another problem is that a large data sample or window of observed transmission energy is required. To obtain such data, the seismic source is required to emit energy in the source well to sensors in the receiver well or wells at depths of several hundred feet both above and below the target reservoir or formation of interest. Because it is rare for wells to be drilled deeper than the formations of interest, this condition is very hard to achieve. Finally, tomographic reconstruction or imaging processes are based on an interwell velocity model. Such a velocity model requires precise positioning or location data for the well, in addition to the observed signal travel times. Thus, accurate location data for the wells is mandatory. And, finally, seismic crosswell tomography is very expensive and lacks horizontal resolution when the well spacings are large. Seismic crosswell tomography is taught, for example, by U.S. Pat. No. 2,231,243 and U.S. Pat. No. 4,214,226.
A method for seismic surveying using seismic waveguides in the earth in which seismic energy is transmitted from outside the waveguide and detected within the waveguide or vice-versa is taught by U.S. Pat. No. 5,260,911. Coupling of energy between the outside and inside of the waveguide is affected by energy leakage at coupling sites where the waveguide departs from planarity. The method analyzes seismic signals to determine the position and nature of coupling sites and the propagation characteristics of the waveguide. U.S. Pat. No. 3,154,760 teaches a method for recording and reproducing seismic waves. U.S. Pat. No. 5,062,084 teaches a method and apparatus for acquiring acoustical data from a borehole using a borehole digital geophone tool which is capable of operating from a standard 7-conductor logging cable with no special cables being required because the individual digital geophone modules which form the tool are connected together by short, separate lengths of the same 7-conductor cable. The borehole digital geophone tool is indicated to be useful in carrying out tomography surveys.
Methods for determining the continuity of a subsurface formation layer located between two vertical boreholes are taught by U.S. Pat. No. 5,005,159, U.S. Pat. No. 4,562,557, and U.S. Statutory Invention Registration H1307. U.S. Pat. No. 3,858,167 teaches an arrangement for determination of the continuity of thickness and of structural-tectonic elements of coal seams in which a transverse seam wave excited by the blast of charges of a directionally oriented blast base is used to determine the continuity of thickness and structural-tectonic elements of coal seams. Direct elements of transverse seam waves and elements reflected from tectonic dislocations are picked up by pick-up devices and, by their character, the conditions of the structural-tectonic elements of the coal seam may be determined. U.S. Pat. No. 2,276,335 teaches a seismic prospecting method for obtaining necessary information regarding the depth of a weathered layer of the earth utilizing the time required for waves to travel vertically through the weathered layer and the wave velocity in the earth just beneath the weathered layer.